Physiological foundations of egg production and methods of poultry sex regulation

Formation of an egg. Egg production is a hereditary trait, and its intensity is largely determined by the physiological processes of egg formation, which are linked to environmental conditions. A bird’s ovary is unpaired: only the left one develops to a functional state. Both ovaries begin to form in the initial stage of embryogenesis, but their development is uneven. As early as in a 4–7-day-old embryo, the left ovary is larger and heavier than the right one. By the end of incubation, the right ovary shows signs of degeneration; it also differs from the left one in structure—it lacks a cortical layer.

When the left ovary is experimentally removed in chicks during their first days of life, the right one grows, becomes similar in structure to a testis, and is capable of spermatogenesis. However, due to the absence of a vas deferens, sperm cannot be released. If the left ovary is removed at a later age, the right ovary develops, and follicles and yolks may even form, but the formation and release of an egg are impossible. In cases of ovarian or testicular degeneration in an adult bird, the hen or rooster acquires secondary sexual characteristics of the opposite sex, including specific plumage, comb shape, voice, etc.

The ability of birds to change sex was noted a long time ago, and Aristotle pondered the causes of this phenomenon. Even in the 20th century, "hens" that crowed like roosters or "roosters" that laid eggs seemed supernatural or bewitched, and their appearance on a farm was said to portend misfortune.

Further study of this issue may be important for understanding the laws of sex formation and regulation. In recent years, some progress has been made in this area. Through the treatment of hatching eggs with hormonal preparations, increasing the methionine content in the diets of breeding hens, and several other methods, attempts are being made to alter the sex ratio in day-old chicks in favor of increasing the number of pullets. However, achieving high productivity from sex-transformed birds requires even more experimental work. If successful, it will be possible for farms specialized in egg production to hatch only pullets, which is of great economic importance.

The ovary is located to the left of the midline of the body, near the anterior edge of the kidney, and is suspended by the serous membrane. There is a large number of oocytes in the ovary: in hens, for example, about 2,000 oocytes visible to the naked eye and more than 12,000 of microscopic size have been counted. However, only a relatively small portion of them reach maturity and develop into eggs. Thus, the known highest egg production of a hen in its entire life is 1,519 eggs. In any case, the potential for increasing egg production is not limited by the reserve of egg cells. Furthermore, as a result of breeding work, the structure of the ovary changes in such a way that the number of egg cells increases. For example, a wild duck has about 500 egg cells in its ovary, while a domestic one has more than 1,000. Fig. 3. Ovary and oviduct of a hen: 1 — ovary with follicles; 2 — infundibulum of the oviduct; 3 — follicle membrane; 4 — albumen-secreting part of the oviduct; 5 — mesentery; 6 — isthmus; 7 — uterus; 8 — large intestine; 9 — cloaca.

The size and weight of the ovary in birds of different species and breeds are not the same, and there are also individual differences. In a growing bird, the ovary gradually increases in size. Egg laying in hens begins at the age of 4–6 months, in turkeys at 7–8 months, and in geese and ducks at 8–10 months. Under the influence of selection and controlled environmental conditions, birds can start laying eggs at an earlier age. During the laying period, the ovary is 10–15 times larger than in the resting period. In a young hen starting to lay, the ovary is 5–6 times heavier than during molting or after egg laying has ceased.

Egg cells are located in the outer, cortical layer of the ovary, in the so-called follicular zone. Each egg cell is contained within a follicle, the membrane of which is connected to the ovarian stroma. The youngest, primary follicles consist of an egg cell without yolk, while yolk gradually accumulates in more mature, secondary follicles. The increase in the amount of yolk is associated with the influx of nutrients through the circulatory system. Follicles do not enlarge simultaneously, but one after another, reaching the size of an egg yolk. Birds belonging to breeds characterized by high egg production, such as Khaki Campbell ducks, have more mature follicles in their ovaries than ducks of meat breeds of the same age, such as Pekin ducks.

The initial size of an egg cell is less than 1 mm; after nine days, the yolk weighs 18–20 g. After ovulation, the blood vessels of the follicle constrict, and the blood flow is significantly reduced. This likely explains the absence of bleeding in the ovulated follicle. At the same time, morphophysiological changes occur in the circulatory system, leading to an increased blood supply to the next follicle preparing for ovulation.

The growth processes of an egg in the ovary are influenced by the hormonal activity of the organism and are regulated by the nervous system. The introduction of pituitary hormones into the body leads to a rapid and simultaneous increase in the size and mass of several oocytes. By influencing the metabolism of poultry through light and certain other environmental factors, it is possible to accelerate the formation of yolks and increase productivity. The acceleration of yolk formation can be genetically determined. At the TSHA Poultry Department, using the heterosis effect, researchers have obtained two eggs from a hen on individual days or very large eggs with two yolks.

The oocyte grows rather slowly. Its growth occurs significantly faster only nine days before ovulation, with the oocyte diameter increasing approximately 6-fold in the final six days. When the conditions of livestock feeding and management are disrupted, the rate of egg formation—and consequently, productivity—decreases rapidly. Degeneration of maturing oocytes is possible. Conversely, the restoration of the normal process of egg formation and oviposition requires some time. Therefore, it is necessary to constantly maintain an environmental regime that meets the requirements of the poultry organism for its high productivity.

Initially, the nucleus occupies a central position in the oocyte, then it begins to move towards the periphery. Yolk is deposited in layers of light and darker colors. Light and dark yolk differ in their physical properties. In the center of the oocyte lies the light yolk, from which the so-called latebra is formed, resembling a flask in shape. The narrow part of the latebra extends to the periphery of the yolk and surrounds the germinal disc.

The color of the yolk is largely due to pigments entering through the blood: carotenoids, especially xanthophyll, and in significantly smaller amounts, carotene. When feeding poultry greens, grass meal, and other feed rich in carotenoids, the yolk color becomes more intense.

Carotene and cryptoxanthin, one of the xanthophyll group, act as provitamin A; other xanthophylls contribute to the efficient use of vitamin A in the body. By the color of the yolk, one can judge its carotenoid content and, consequently, the vitamin value of the egg, which characterizes its nutritional and incubation qualities.

During the process of egg formation, as the yolk mass increases, the relative amount of water in it decreases. The egg is enriched with fats, proteins, minerals, and vitamins. In the final growth phase, an elastic vitelline membrane forms on the surface of the yolk under the follicular membrane, through which nutrients continue to enter the oocyte.

When the yolk reaches its full size (about 35—40 mm in diameter), ovulation occurs. The yolk is released from the follicle due to the rupture of its membrane along the stigma (the thinned part of the follicular membrane facing the body cavity) and enters the infundibulum of the oviduct adjacent to the ovary.

Yolk formation and the process of ovulation are linked to metabolism, regulated by the nervous system, and are significantly influenced by environmental factors. Good feeding and management conditions contribute to the rapid growth of oocytes.

The oviduct of a bird is a long, convoluted, highly elastic tube, the anterior end of which opens into the body cavity near the ovary, and the other into the cloaca. The diameter of the tube varies in different parts, but it can expand as the egg passes through. The mesentery on which the oviduct is suspended allows for its significant movement.

Depending on the physiological state and productivity of the bird, the size and mass of the oviduct vary greatly:

• in a non-laying hen, the length of the oviduct is about 15 cm;
• during intense oviposition — 65 cm or more, and the width increases from 0.5 to 10 cm.

The oviduct is divided into the infundibulum, the magnum, the isthmus, the uterus, and the vagina. The length of these parts in a hen is shown in the table:

In a hen that has ceased productivity, the infundibulum, uterus, and vagina decrease by 2.5—3.5 times, and the magnum by 6 times.

The wall of the oviduct consists of several layers: the outermost is the serosa, followed by longitudinal muscles, connective tissue with a large number of blood vessels, circular muscles, and a mucous membrane with intensely developed blood vessels and glands, forming folds of varying sizes. The mucous membrane is lined with ciliated epithelium. The oviduct is intensely innervated. This structure of the oviduct facilitates the process associated with egg formation and its movement.

The secretory function of the oviduct is carried out by the epithelial cells and tubular glands of the mucous membrane. The secreted substance differs in physical properties and chemical composition. It is possible that liquid egg white is formed from the secretion of tubular glands, while the secretion of cuboidal cells — mucin — is part of the dense egg white. Most of the egg white is created in the glands of the magnum. A layer of dense egg white is deposited around the yolk first. Due to the peristaltic movements of the oviduct walls and their folding, the egg performs rotational movements along its longitudinal axis. The dense egg white at the blunt and sharp ends of the egg forms spiral structures, or chalazae. The chalazae hold the yolk in the center of the egg. As the egg moves further, a layer of medium-dense egg white appears, which consists of a fine network of mucin fibers. Liquid egg white gradually accumulates between them. The rotation of the egg during-

Fig. 4. Histological structure of a hen's oviduct (cross-section):

left — albumen-secreting part of the oviduct; 1 — serosa; 2 — outer layer of longitudinal muscle fibers; 3 — connective tissue with vessels; 4 — layer of circular muscle fibers; 5 — second layer of the mucous membrane; 6 — mucous membrane with glands;

7 — surface epithelium; right — mucous membrane of the albumen-secreting part of the oviduct; 1 — mucous membrane with glands; 2 — surface epithelium; 3 — lumen of the oviduct (according to M. Ya. Solovey and Z. M. Davydova).

leads to the secretion of liquid albumen inside the dense one and the appearance of a middle liquid albumen layer adjacent to the inner dense layer.

The shell membranes are formed in the isthmus, consisting mainly of protein secreted by the glands of this part of the oviduct. The egg arriving here has only 40—50% of the required amount of albumen. The outer layer of liquid albumen is supplemented in the isthmus and uterus, when its membranes are still very porous. At this time, inorganic substances dissolved in water also continue to enter the egg.

The shell is formed in the uterus. This process initially proceeds slowly. Only individual deposits of calcium appear on the surface of the egg, which gradually increase. Among them are small amounts of organic substances, mainly of a protein nature. In this way, the mammillary layer of the shell is created. The base of each mammilla is connected to the shell membrane. The mammillae gradually increase, their side walls come into contact with each other, but small pores remain between them, through which air penetrates into the egg. The glands of the anterior part of the uterus secrete

Fig. 5. Egg in the hen's uterus (radiogram according to S. I. Smetnev and protein, which in the form of fibers

M. Davydova). is located on the mammillary layer.

A — longitudinal section: 1 — shell membrane; 2 — spongy layer; 3 — mammillary layer; 4 — shell membrane; 5 — pore. B — cross-section through the mammillary layer: 1 — organic matter; 2 — mineral matter.

Crystals of the secreted calcium salts soon fill the space between the network of protein fibers; this creates a dense and strong spongy layer of the shell, which makes up the majority of it. The spongy structure of the layer becomes visible only when calcium salts are removed. Pores are also formed in this layer, opening on the surface and on the inner side of the shell.

A large amount of mineral substances is secreted in the uterus, mainly calcium carbonate (about 5 g per 20 h) and significantly less phosphorus. Before and during egg-laying, the calcium content in the blood of laying hens increases by 2—3 times; the amount of phosphorus also increases. If there is a lack of calcium in the feed, the organism mobilizes it from the skeleton. If this proves insufficient, the bird lays eggs without a shell and, as a result of metabolic disorders, may stop laying eggs.

The time of egg formation in the oviduct varies significantly for an individual bird and changes in the same species during different seasons of the year under the influence of environmental conditions.

In hens that lay eggs daily, ovulation occurs approximately half an hour after the egg is laid. The yolk moves through the albumen-secreting part of the oviduct within 3 hours, is delayed in the isthmus for a little more than an hour, and spends the longest time (19 hours or more) in the uterus. The expulsion of the egg from the uterus happens very quickly. In hens, the difference in the time of egg formation is explained mainly by a greater or lesser delay in the uterus. If about 24 hours are spent on egg formation, the hen lays daily; if more than 24 hours, the hen lays with intervals, since ovulation does not occur in the afternoon. The appearance of intervals is therefore explained by a delay in ovulation, probably associated with the genetic characteristics of the bird, the reaction of the bird's nervous system to environmental conditions, and changes in metabolism during the day. A hen whose egg is formed in 26 hours, for example, lays in the following days around 10, 12, 14 hours and takes a break. The more time spent on egg formation, the fewer eggs the hen lays in a row. Consequently, when studying egg production, its cyclicity becomes evident.

An egg-laying cycle is called the number of eggs laid by a layer without an interval. The size of the cycles is to a large extent an inherited feature of the bird. Cycles can range from one to several dozen eggs. Between cycles, intervals are formed, expressed in the number of non-productive days. The longer the cycle, the shorter the interval, and vice versa. Long cycles with short intervals characterize good laying hens, short cycles with long intervals — poor ones. During long cycles, egg laying occurs at almost the same hours, with the exception of a few days at the beginning and at the end of the cycle. Cases of a hen laying two eggs in one day, in the morning and evening, have been noted. In the same laying hen, cycles tend to repeat, although they change slightly in connection with age and environmental conditions.

The rhythm of egg-laying refers to the frequency of cycle repetition. At the TSHA experimental station, they keep records of the time hens lay eggs.

Table 2. Egg-laying time and monthly egg production of hens

Table 3. Average cycle size and time of egg formation in relation to the time of laying and monthly egg production

Hens Time spent on Average cycle Number Group Eggs laid per formation of size (days) M + m month (pcs.) one egg 1 1 29 29 + 0.00 24.0 2 > 28 8.3 = 0.90 26.6 3 » 28 8.8 + 2.53 26.7 4 » 27 6.5 + 0.65 21.6 5 n 15 4.0 + 0.36 300 ' 6 y 24 3.4 = 0.24 30.6 7 » 24 4.0 + 0.45 29.6 8 w 21 2.1 = 0.10 34.8. 9 » 21 2.0 + 0.00 36.0 10 r 20 2.2 + 0.22 35.5

Reviewing Table 2, it is easy to note that some hens lay at almost the same hours and their egg production is the highest. Less productive hens lay later each day. These features of egg production are reflected in its cyclicity and are related to the time of egg formation. |

Comparing the data in Tables 2 and 3, it can be noted that the first four hens, which laid the highest number of eggs per month, spent less time on egg formation, and their egg-laying cycle was longer. Other hens (Nos. 5, 6, 7 and 8, 9, 10) were characterized by a longer duration of egg formation, shorter cycles, and consequently, lower egg production. The difference in indices between the groups is statistically significant. If standards or feeding regimes and housing conditions are disrupted, the rhythm characteristic of a given bird or its group changes. For example, at an air temperature below 07 C, cycle sizes shorten and intervals increase. As a result, egg production decreases.

The importance of egg-laying rhythm, cycle size, and intervals is very significant and characterizes the productivity of the poultry. If we consider egg production as a physiological process, the application of all basic zootechnical methods and techniques aims to reduce the period of egg formation and increase egg production.

Sexual maturity is an inherited trait and is determined by the age of the bird at the time of laying the first egg. In hens, sexual maturity occurs at the age of 120—180 days, in turkeys — 200—250, in ducks and geese — 250—300 days.

Sexual maturity of individual hens and, in particular, the flock is also expressed by age in days from hatching when the bird's egg production reaches 50%. For example, 1,000 hens lay 500 eggs per day.

It changes under the influence of the hatching time, breeding work, and feeding and housing conditions. Birds of egg-laying breeds are earlier maturing than meat breeds, although individual differences are very large.

Hens possessing similar, genetically determined sexual maturity are capable of starting egg-laying at approximately the same age under favorable conditions, but the development of the ovum depends on feeding and housing conditions. Cage-kept hens, under a uniform regime of air temperature, lighting, and feeding throughout all seasons of the year, begin laying at approximately the same age, and their annual egg production does not depend on environmental factors.

When keeping poultry on the floor, the timing of the onset of egg production depends on the hatching season:

• pullets hatched in March-April begin to lay in September—October;
• pullets hatched in September — in March—April.

In large specialized farms, the flock is stocked with young hens hatched in different seasons of the year, which allows for obtaining many eggs throughout the entire year.

Egg production in the first 3—4 months is in direct correlation with sexual maturity. In normally developed hens of egg-laying breeds, annual egg production correlates with egg production for the first three months; in hens of dual-purpose breeds, which are somewhat less early-maturing, this ratio is not sufficiently reliable. The annual productivity of this type of hen can be judged by the productivity of pullets during the first four months of egg-laying. These data are used for accelerated evaluation of hens for productivity and breeding qualities during selective breeding and pairing.

The earlier hens begin to lay, the more eggs they produce in the first months, for the year, and for the entire biological egg-laying period, provided, of course, that feeding and housing conditions are favorable. However, hens with excessively forced sexual maturity and a disturbed proper ratio of growth and development initially lay very small eggs, and their egg production is unstable.

Breeding work is aimed at developing lines and producing hybrid hens of robust constitution with high sexual maturity, laying the largest possible eggs from the beginning of egg-laying and in total for the year.

Egg-laying persistence is an inherited quality. It is expressed in the bird's ability for rhythmic egg-laying from the time of reaching sexual maturity until the cessation of laying and molting. In spring-hatched hens with persistent egg-laying, its cessation and molting usually occur in the autumn of the following year. Hens that lay 365 eggs per year without interruption possess maximum egg-laying persistence, but such hens are very few.

In practice, a laying hen is considered to have stable productivity with the following characteristics:

Lower stability of egg production depends both on the genetic characteristics of the poultry and on breaks in laying that occur during broodiness and molting. Broodiness is an unconditioned reflex. During evolution, wild birds developed an alternation of biological processes that determine the reproduction of the species:

• Spring egg laying;
• Subsequent brooding.

Nowadays, when incubation of eggs plays a huge role in poultry farming, one should strive to reduce or even completely suppress the instinct of broodiness to increase the productivity of the poultry. I. P. Pavlov pointed out that instincts can change under the influence of directed human efforts. Indeed, as a result of long-term breeding work, egg-laying breeds have been developed that almost never go broody. The instinct of broodiness as an unconditioned reflex is suppressed in them and, as a rule, is almost never inherited. Hens of non-broody breeds are distinguished by the highest egg production.

Molting in wild birds is seasonal. They completely molt by autumn, by the time of migration or the onset of the winter period. Molting is a biological adaptation of the bird to life conditions. Poultry has retained this feature of its wild ancestors. Egg production and molting are influenced by many factors and are regulated by the nervous system of the bird.

For a poultry farmer, the beginning of molting is the main signal to change the productive period of the flock. Although externally this process looks like ordinary feather loss, its biological basis lies in the growth of new plumage. Molting and egg laying are physiologically mutually exclusive, so the bird cannot simultaneously lay eggs and change feathers.

The relationship between the ovaries and the thyroid gland of the bird

New feathers are formed from special feather papillae, the development of which is triggered by a thyroid hormone. However, during egg production, the bird's ovary releases hormones that block the activity of the thyroid gland. This mechanism prevents the bird's body from spending nutrients on feather growth during the period of active egg laying.

As soon as the productive period ends, the hormonal activity of the ovary decreases sharply. The thyroid gland is freed from this inhibitory influence and begins to actively stimulate the development of feather follicles. As a result, new feathers grow rapidly and push the old plumage out.